Differential interferometers are well known in the art as metrology tools useful in a variety of scientific and technological fields, for example, biology, geology, forensics, nutrition science, medicine, and semiconductor processing. Differential interferometers are instruments that measure the step height between two regions on a sample or the surface profile of a sample by measuring the interference of an electromagnetic beam reflected from the two different heights.
FIG. 1 is a schematic representation of a conventional interferometer 10. As shown in FIG. 1, the interferometer 10 includes a laser light source 12 that generates a collimated monochromatic light beam. Modulator 14 then modulates the light polarization state at a frequency Ω, and consists of a rotating polarizer or a polarizer and elastic modulator combination. The light then passes through a beam splitter 16 and is divided by a Wollaston prism 18 into two components, s and p. A lens 20 is positioned to focus the two beams on the surface to be measured, e.g., step surface 22. The beams are reflected back through the lens 20 and recombined by the Wollaston prism 18 before being deflected by beam splitter 16 through polarizer 30 (analyzer) and to detector 32. A conventional metrology tool 10 may include other various systems such as auto-focusing and viewing optics (not shown).
Detector 32 measures the intensity of the light as a function of time as shown in FIG. 2. The phase change Ωh, of this function from a measurement of a flat region and a step region, as shown in FIG. 3A and 3B, is proportional to the height of the step as given by equation:                     h        =                                            4              ⁢              π                        λ                    ⁢                      φ            h                                              eq        .                                   ⁢        1            where h is the height of the step, and λ is the wavelength of the light. However, if the material on top of the step is different from the material on the bottom of the step, e.g., the top can be a stack of transparent layers, whereas the bottom can be metal, the wavelengths are reflected in different phases due to of the different complex reflectivity coefficients of the two different materials. This difference must be taken into consideration when calculating an accurate step height of the sample. Therefore, the phase θ, measured by the interferometer when the material on the top and bottom of the step differ becomes:θ=φh+φmat1+φmat2  eq.2where φmat1 and φmat2 are the phase values due to the reflectivity coefficients of the material on the top of the step, material 1, and the material on the bottom of the step, material 2, and φh is the phase difference due to the height of the step. Therefore, because the step height is desired, it becomes necessary to determine the reflectivity coefficients of the two materials.
An interferometry measurement would be sufficient without determining the reflectivity coefficients of the two materials if there were an external absolute reference, for example, where one of the beams is reflected back from a mirror, as in a Michelson interferometer. However, the disadvantage of using an external absolute reference in a metrology system is that such systems invariable suffer from a high degree of sensitivity to vibration.
Another proposed approach to measure the reflectivity coefficients is the use of a spectroscopic reflectometer. The reflectance spectrum can be analyzed to determine the structure and optical properties of the measured material, and this information can produce an optical model to calculate the phase quantities. A disadvantage of this approach is that completely different equipment or hardware is required to measure φmat1 and φmat2, thus increasing the cost and complexity of the step height metrology system.
The use of multiple instruments in many industries, such as semiconductor manufacturing, is undesirable because of the high fabrication and maintenance costs of clean rooms or other operation environments. Further, the use of multiple instruments creates time-consuming transfer of samples from one instrument to another and reduces the accuracy of the instruments because it is difficult for multiple instruments to measure from the exact same area of the samples.
Furthermore, a reflectometer is a tool that is not sensitive to phase. The process of indirectly determining φmat1 and φmat2 by reproducing the structure from a reflectance spectrum may propagate systematic errors.
Thus, in general a metrology tool that can accurately measure the step height of a sample, including a transparent layer or two different materials, is desired. Further, a decrease in the size of the metrology tool as well as reducing the cost and maintenance associated with the metrology tool is desired.